1. Field of the Invention
The present invention relates to thermographic recording films including a washcoat. The subject washcoat comprises a compound containing at least two epoxide moieties and a quaternary ammonium salt including at least one hydroxyl functional group.
2. Description of the Related Art
Thermographic recording films are well known in the art. Examples of such films are disclosed in U.S. Pat. No. 5,278,127 to Dombrowski and McPherson, U.S. Ser. No. 08/179,516, U.S. Pat. No. 4,904,572 to Dombrowski, Jr. et al. and U.S. Pat. No. 5,198,406 to Mack et al., all assigned to the assignee of the subject invention. Such films comprise multi-layered elements which include a support carrying a image-forming system and a protective topcoat layer positioned thereover. The image-forming system of such films includes a di- or triarylmethane thiolactone dye precursor, a Lewis acid material, an organic acidic material, and a binder. The image-forming systems of such films are well known and are described in the aforementioned prior art patents along with U.S. Pat. Nos. 4,970,309 and 5,220,036 both to King, and U.S. Pat. No. 5,196,297 to Dombrowski, Jr. et al., and European Application No. 250,558 to Dombrowski, all of which are assigned to the assignee of the subject invention.
The topcoat of such films comprise a layer or layers coated upon the image-forming system for protecting the system from mechanical wear, e.g. "gouging". "Gouging" results in actual depressions or indentations in the recording film which can be either continuous or intermittent. Gouging is believed to be caused by high temperatures, pressure and/or sticking. The topcoat must additionally permit heat to pass therethrough, and in certain applications, must be transparent. Topcoats typically include a binder, silica, lubricants, and are generally at least 1 micron thick. By way of example, U.S. Pat. No. 4,898,849 to Kang discloses a thermographic film including a crosslinked protective topcoat having fluorocarbon lubricants thereon.
In addition to topcoats, washcoats may also be utilized with such films. Washcoats are layers coated upon the topcoat to enhance topcoat properties, particularly surface properties.
A variety of challenges are encountered with such thermographic recording films. In addition to gouging as described above, head build-up and streaking are common problems. "Head build-up" is the build-up of components of the thermographic recording film on the thermal print head. Head build-up can cause streaking in the printed image, decreased image density with continued printing and damage to the thermal print head. Head build-up can become so pronounced, particularly when a lubricant, e.g. polytetrafluoroethylene, is present in the topcoat, that it appears as "spider webs" on the thermal media and printer.
"Streaking" is believed to be the result of the insulating effect of head build-up on the printing element(s) of the thermal print head which interferes with printing causing linear cross-web non-uniformity ("streaking") in the printed image.
The presence of a lubricant in the topcoat is generally desired to impart slip characteristics and to decrease gouging of the printed image, however, head build-up usually becomes more pronounced when a lubricant, e.g. polytetrafluoroethylene, is used in the topcoat. Generally, the greater the concentration of lubricant, the greater the degree of head build-up.
In addition to gouging, streaking, and head build-up, the films as described typically have problems associated with static energy charge build-up. In this regard, many known anti-stats may be used for reducing static energy charge build-up. Such anti-stats include quaternary ammonium salts which are typically compounded into the supports of such films (typically referred to as "bulk anti-stat") or provided as a separate layer adjacent to the support. For example, U.S. Pat. No. 4,711,816 to Wittnebel discloses a sheet material for an electrostatic copier which includes a layer of electrically conductive material. The conductive material may include long chain amines, amides and quaternary ammonium salts such as stearamidopropldimethyl-beta-hydroxyethylammonium nitrate (CYASTAT SN). Similarly, U.S. Pat. No. 4,480,003 to Edwards et al. discloses sheet material for electrostatic copiers which include a conductive layer for dissipating electrical charge build-up. The conductive material may include materials such as epoxy silane/silane sulfonate resin.